Bruno Loff:
To answer number 1, I suggest you take the sensation of "diffuse soft humming" as a whole, and do vipassana or whatever on that. One thing that has helped with my practice is this: there are various kind of focus: local/global, hard/relaxed, etc; if one kind of focus isn't moving things along, then try a different kind of focus.
With regards to the cool current, in yoga this is described as the result of kundalini rising up through the left channel (ida, I think). Then you get cold. You should be careful with this as if there is a serious unbalance you can become really really cold. If it goes through the right channel (pingala) it will feel really really hot. A teacher once recommended that in these extreme situations one should cover the body or take a cold shower, respectively. If kundalini goes through the center channel (sushuma), it will feel like bliss/sexual ecstasy (these three channels are all located inside the spine and are hard to distinguish).
I would suggest you try doing 5-10m of the microcosmic orbit before each sit (look up on the internet to see how), it should normalize things.
Thanks, Bruno.
Your comment on the kundalini is interesting. I shall look into the practice you suggest.